82 THE ANCESTRY OF VERTEBRATES 



number of myotomes exceeds that of the gill-slits but 

 this is only a secondary phenomenon again. 



In Necturus Miss Platt (1894, 1897) found a complete 

 accordance of mesomerism and branchiomerism, the second 

 gill-slit is situated beneath the auditory invagination, in 

 front of the first meta-otic somite, the third one between 

 Ihe first and the second post otic somite, eic. The same 

 seems to hold for Gymnophiones, to judge from MARCUS' 

 (1910) figures and description, though certain corrections in 

 his interpretation of the somites seem necessary. Of the 

 post-otic head-somites the last one still produces muscles 

 of a permanent nature, while also the last but one still pro- 

 duces a few fibres. GOODRICH (1911, p. 116), working on the 

 axolotl, fully confirms Miss Platt's conclusions regarding 

 Necturus. 



If now in lower Vertebrates we find such an undeniable 

 correspondence between branchio- and mesomerism, it is 

 ^ hard to deny or doubt such a correspondence in higher 

 > groups where the mesomerism in this region has become 

 I \i^ss distinct when the gill-slits appear. 

 ^~~^, Paired intestinal dive, ticula among Protostomia. — For the 

 solution of the problem of the relation of the branchio- 

 merism to the mesomerism we look in vain to the Annelids. 

 Neither here, nor in other Protostomia, do gill-slits occur 

 and it need not be further argued, that there can be any 

 question of a airect comparison to the gill-slits of Bolano- 

 £lossus. Among the parenchymatous worms, however, we 

 often find paired diverticula of the gut which remind us of 

 Ihe gill-pouches of Vertebrates. They often alternate quite 

 regularly with the equally paired genital follicles, from 

 which Meyer (1890) derives the coelomic segments of 

 Annelids. This is the c^se in several Turbellaria and in 

 most Nemerteans. In Polyclads the ends of these diver- 

 ticula sometimes apply themselves to the ectoderm, thus 

 giving rise e g. to the villi of forms like Thysanozoon, 

 which no doubt have a respiratory function. In other forms 

 again they may even open to the exterior (Yungia). 



Something quite similar is found among Molluscs in 

 the Nudibranchs, which in several respects show such a 

 remarkable resemblance to Polyclads. Here also we have 

 paired diverticula of the gut, applying themselves to the 

 ectoderm and giving rise to the gills or cerata, which often 

 open to the exterior. In the year 1845 NORDMANN gave a 



